1. Field of the Invention
This invention relates to a precision positioning device. In particular, the invention relates to a multi-stage module capable of displacing an element to be aligned in different planes extending along perpendicular but coplanar axes, so that the planes can be articulated relative to one another at any desired position within a desired range.
2. Description of the Related Art
Alignment stages are known and one may find their application in numerous and diverse fields. Typically, alignment stages are utilized for the positioning of elements, and are required to precisely position an element to be aligned in a desired position and to repeat the desired position at a high rate.
As a rule, an alignment stage includes numerous mechanical linkages translating different motions necessary to displace the element to be aligned in different planes. The alignment stage can be a single tilt stage, in which the element to be aligned is angularly displaceable about a single axis, or a dual alignment stage providing displacement of the element about two axes extending transversely to one another.
Typically, two actuators mounted on a dual alignment device displace a single frame along transverse axes. Known structures of such dual alignment devices have the actuators mounted in different planes, thus rendering the overall structure of the dual stage rather complicated and cumbersome.
There are, however, known multi stage alignment devices utilizing multiple actuators, which have respective axes extending in a common plane. One of such multi stage alignment devices is manufactured by xe2x80x9cPhysik Instrumentexe2x80x9d (referred further as a PI device). The PI device has a single frame selectively tiltable about perpendicular and coplanar axes in response to actuation of two piezoelectric actuators (PZT).
While PZT drives are accurate, their structure may cause certain difficulties for the overall construction of the multi-stage alignment device. One of the known disadvantages of the PZT actuator includes a complex actuating assembly having at least one motion-translating component which converts axial displacement of the PZT actuator into angular displacement of a tilt frame supporting an element to be aligned. Thus, since the PZT drive moves along a drive axis extending perpendicular to a pivot axis of the tilt frame, precision with which the tilt frame pivots can be compromised.
Furthermore, the PZT drives utilized in the PI device are typically energized by very low voltage signals. To generate such low voltage signals, a power source has to meet certain requirements, which contribute to the relatively high cost of the PI device. Also, the PZT drives are open loop devices designed to displace a tilt frame at a very small angle, which is difficult to detect in the open loop design. As a consequence, positioning sensors mounted to verify the frame displacement can add to structural complexity and cost of the PI device.
It is, therefore, desirable to provide a precision positioning tilt device having a simple and reliable structure. Also, a precision positioning tilt device provided with a compact structure actuated by two independent closed loop servo control actuators is desirable as well. Furthermore, a precision positioning tilt device characterized by a direct mechanical link between each tilt stage, a respective servo torque motor and a respective positioning sensor is also desirable.
The above and other objects are attained by a precision positioning tilt device having a small size, which allows a high accuracy of positioning an element to be manipulated by displacing the element in two planes, which extend along mutually perpendicular, but coplanar axes. The inventive precision positioning tilt device is a limited pivotal rotary device for small 0-5xc2x0 angular motion, whose use is determined by a particular practical application of the device and can include, but is not limited to, aligning surgical instruments, connecting optical components, and other operations requiring high precision positioning of elements.
The inventive precision positioning device includes an inner tilt frame operable for pivoting about a frame axis and actuating both the element to be manipulated and a platform mounted on the tilt frame to pivot with the tilt frame about the frame axis. Furthermore, the platform is operable for pivoting independently from the tilt frame about a platform axis extending transversely to the frame axis. To facilitate kinematics of the inventive precision positioning device, the frame and platform axes are coplanar providing all pivotal components of the inventive positioning device with a common geometrical center of symmetry positioned at the intersection of the axes.
In accordance with one aspect of the invention, first and second electric DC brushless servo torque motors directly actuate the tilt frame and the platform, respectively, without assistance of transitional mechanical linkages. Each of the first and second motors can be energized independently from the other motor so that the precision positioning device, in accordance with the invention, can be utilized as a single or dual tilt stage module. The use of closed loop servomotors affords high resolution and the ability to achieve single count moves with positive feedback. Another advantage of the closed loop servo system is the accommodation of various load conditions and operating modes, such as various speeds. Furthermore, the closed loop servo system is characterized by the high torque levels of the DC motors allowing fast positioning of the element to be manipulated in a desired position which is steadily registered and, if necessary, can be repeated at a high rate.
In accordance with another aspect of the invention, position feedback is provided by installing grating scales directly on the movable tilt frame and platform and having non-contacting solid-state sensors juxtaposed with the grating scales. The grating scales are fixed to and, thus, pivot synchronously with the platform and the inner tilt frame, respectively, so the measurement provided by the solid-state sensors corresponds to true displacement of the movable frame and platform.
According to still another aspect of the invention, the precision positioning tilt device has a compact and rugged design including a cavity provided in the platform and dimensioned to receive the second DC brushless motor so that a shaft is centrally positioned within the platform and fixedly mounted on the inner tilt frame. Thus, the inner tilt frame serves both as an actuator for the platform as a result of a rigid linkage between the inner tilt frame and platform, and as a support for the second motor.